The present invention relates to a method of plastic deformation of crystalline materials.
More particularly, it relates to a method of plastic deformation of metals, alloys and other crystalline materials which allows controlling of structure and texture of such materials.
Controlling of structure and texture of materials is one of the most important ways to improve properties of metals and alloys. Processes of plastic deformation are very important in a general cycle of thermal mechanical treatment, including various combinations of thermal and mechanical action upon the material to be worked. Objectives of structure formation or in other words of regulation of size and shape of grains and development of a complicated internal grain substructure are versatile. Sometimes the problem is a substantial reduction of grain size in one direction in tenths and hundreds thousands times and formation of a laminated structure. For example for copper-niobium composites manufactured in situ, it is possible with this approach to obtain superhigh strength of more than 2,000 MPA of a conductive material. However, with known technical means these results can be obtained only for very thin bands and foils with a final thickness of approximately 0.01 mm. In other cases it is necessary to provide multiple reduction of the grain size in two directions with substantial increase of their size in a third direction, which results in formation of fiber structures. For example a method of producing high strength and ductile thin wire of tungsten is known by substantial drawing with a gradually reducing temperature from sintered brittle workpiece. As for obtaining of such results for articles having great masses, this is now practically impossible. In certain situations however it is necessary to obtain the exact correspondence between maximum and minimum sizes of grains (aspect-ratio). On the other hand, for great variety of objectives, the plastic deformation is used for development of greatly deformed, but equiaxial grain structures. Thereby it is possible to obtain sub-micronic and nano crystalline structures for many industrial alloys, which have high strength and ductility. One of the important technical applications of this effect is elimination of brittleness of intermetallic alloys at room temperature.
The above described structural changes during plastic deformation are usually accompanied by development of corresponding textures, or in other words predominantly crystalline orientation of grains. Strong texture is a main factor which determines high characteristics of magnetic materials, or strength and toughness of titanium alloys. However, similarly to the structure formation, there are substantial difficulties in controlling texture with known technical methods.
In order to form different types of structures and textures, specific methods of plastic deformation are utilized. The laminated structures and corresponding complete textures are formed by flat rolling as disclosed for example in U.S. Pat. Nos. 3,954,516, 4,080,715, 4,406,715, 4,609,408, 4,722,754. Fibrous structure and corresponding axial textures are obtained during pulling of a material in one direction by axis-symmetrical drawing, pressing and rotary forging as disclosed for example in U.S. Pat. Nos. 4,336,075, 4,511,409, 5,074,907, 5,145,512, 5,120,373. Equiaxial structures and full textures are developed during twisting and special sequence of forging operation as disclosed for example in U.S. Pat. Nos. 3,645,124 and 5,039,356. Equiaxial structures in a textureless material can be made during a multi-stage forging with equal squeezing in three mutually perpendicular direction as disclosed in U.S. Pat. Nos. 3,954,514, 4,466,842 and 4,712,537. The above described methods have substantial disadvantages, in particular as follows:
--High specialization of each method of deformation in development of one specific type of a structure and texture. Thus, rolling is specific for production of laminated structures and full textures, while drawing is specific for obtaining fibrous structures and axial textures. For this reason corresponding types of textures are called textures of rolling and drawing. Other types of structures and textures cannot be obtained by means of these methods.
--There are strict limitations with respect to a geometric shape of material in which a certain type of structure and texture is produced. For example, laminated structure and full texture is obtained in sheet and bands, however, it cannot be made for workpieces of round or square cross-section. On the other hand fibrous structure and axial texture are natural for round cross-sections, but cannot be reproduced for flat sheets and plates. Similarly, a non-textured material with heavily deformed equiaxial structure can be obtained only for articles with small difference in side ratio, in other words close to cubic articles.
--For substantial change of structure and texture of a material to be worked it is necessary to significantly change its shape which is characterized by reduction of its cross-section or in other words a ratio of the area of cross-section of an initial workpiece to a final article. The achieved results increase when the reduction is increased, which in many cases must be tenths and hundreds and sometimes thousands times. Therefore when known methods of plastic deformation are used, high series of properties can be obtained only for articles of substantially small cross-sections such as sheet, foil and thin wire while for enough massive articles the level of properties are always lowered.
--Large non-uniformity of strains and deformed conditions during the processing, which substantially reduces properties of the articles.
--Necessity to use significant reductions leads to high working pressures and forces such as for example for extrusion, or to high labor consumption and time of working such as for example in the case of multi-stage rolling, drawing and forging.
The closest process type to the present invention is a method of equal channel angular extrusion with the use of deformation of simple shear as a metalworking process. The method is proposed by the applicant and disclosed in the inventor's certificate of the USSR number 575892 of Oct. 22, 1974. Some elements of the method are disclosed in publications:
1! Segal V. M. and others. "Plastic Working of Metals by Simple Shear." English translation: "Russian Metallurgy", No. 1, pp. 99-105, 1981.
2! V. M. Segal. "Working of Metals by Simple Shear Deformation Process." In "Proceedings 5th Aluminum Extrusion Technology Seminar", vol. 2, pp. 403-406, Chicago, 1992.
3! V. M. Segal. "Simple Shear as Metalworking Process for Advanced Materials Technology". In "Proceeding First International Conference on Processing materials for Properties", pp. 947-950, Hawaii, November, 1993; and also in the inventor's certificates of the USSR numbers 492780, 780293, 804049, 812401, 902884.
This method is illustrated in FIG. 1. The tool is a die set 1 having two intersecting channels 2 and 3 with an equal cross-section D. The initial workpiece 4 is lubricated and has approximately the same cross-section D. It is placed into the first channel and under the action of plunger 5 is pressed out into the second channel. During this process a deformation is performed by a simple shear with high intensity along the plane of intersection of the channels A--A. When the plunger reaches its lower position B--B, it retracts and the workpiece is removed from the second channel. Therefore the whole volume of the material with the exception of relatively small ends can be uniformly and intensely ready-formed without changing the area of cross-section of the initial material. The above mentioned process can be repeated many times in the same tool, so that extremely high equivalent deformations can be obtained in great articles. Moreover, the process is characterized by low pressure applied to the instrument and small working forces.
The method of equal channel angular extrusion eliminates some above mentioned disadvantages of the traditional methods of plastic deformation. However, it has been recognized that the advantages of the method are obtained in the cases when the final effect of plastic working is determined only by the total quantity of accumulated deformation as for example in the case of break-down of cast metal, strain hardening, consolidation of porous metals, and some others. In the cases when the effect of plastic working is connected with control of structure and texture, there is indefiniteness of the results which sometimes become even negative. Therefore, in addition to the known method of equal channel extrusion it is necessary to develop a special process of its realization, which eliminates the above mentioned controversy and provides principally new possibilities of controlling structure and texture during a plastic deformation.